To solve this problem, the shoulder has an active stability system called the rotator cuff. Four muscles that fan out from the humerus continually pull it back into the socket so that the joint can't dislocate. Around the ball of the humerus, the tendons form a nearly continuous band like the cuff on a man's shirt.
The rotator cuff system is in many ways a miracle of biomechanical engineering. The tendons are relatively compliant and stringy, like a rope, and bone is hard and porous, like cement. Some of their mechanical properties differ by a factor of a hundred-fold or more.
"Attaching a compliant material like tendon to a relatively stiff material like bone is a fundamental engineering challenge," says Thomopoulos.
Merging tendon with bone
Nature has solved this problem by grading the mechanical structure and stiffness of the tissues across the interface between the tendon and the bone.
In an infant, both the bone and the tendon consist of collagen, says Thomopoulos. "Soon after birth, the bone starts to mineralize and a linear gradient forms between the bone and the tendon that slowly stiffens the material and transitions it from hard bone to compliant tendon."
The difficulty, says Thomopoulos, is that this unique transitional tissue is not recreated after injury.
"If you fall off a ladder and your rotator cuff tears, even if the surgeons goes in and puts the tendon back on the bone, he's putting the tendon right against bone. You don't have the graded interface. Biologically, it doesn't reform."
As a result, the failure rate for this surgery can be quite high as high as 94 percent in one study co-authored by Galatz. Many of the patients in the study she published experienced relief from pain, but ultrasound revealed the cuff tear had reopened o
|Contact: Diana Lutz|
Washington University in St. Louis